1. Field of the Invention
The present invention relates to a recording apparatus for recording various kinds of information signals, such as video signals, speech signals and control signals, on an optical disk as an optical recording medium, and a reproducing apparatus for reading and reproducing recorded signals from such an information recording medium. More particularly, this invention relates to an optical disk information recording apparatus and reproducing apparatus, which use an optical head defining recording points or reading points on a recording medium with a plurality of light beams to record or read information signals on or therefrom.
2. Description of Background Information
Recently, it is contemplated to record and reproduce a high-definition television signal on and from an optical disk in the form of a digital signal. One of the features of this signal is the amount of signals to be transferable per unit time, which is 50 Mbps (bps represents the amount of information bits (bits) transferred per second). Since the amount is very large, about 40 times the transfer amount of a currently used compact disk (1.2 Mbps), a relatively high information transfer rate is required.
There is a recording apparatus for recording information on an optical disk of a phase change type as a conventional optical disk information recording form, as shown in FIG. 1.
In FIG. 1, light emitted from a laser diode 1 becomes parallel light by a collimator lens 2. The parallel light passes a beam splitter 3, and is focused on a disk D by an objective lens 4, forming pits on the recording surface of the disk D. The reflected light from the disk D passes the objective lens 4 to become parallel light, which is reflected by the beam splitter 3 to enter a cylindrical lens 5. The light, which has caused astigmatism by the cylindrical lens 5, enters a photodetector 6, yielding a tracking signal and a focus signal.
With this conventional structure, information is recorded in a single spiral form starting from the inner track using a single write beam, and recorded information is reproduced on the single spiral track using a single read beam. Although it is typical to use three beams for control to trace a single track, only one light beam is used for information reproduction.
In recording information in a single spiral form with a single beam, the number of rotations of the disk D should be increased and the information density should also be improved in order to increase the transfer rate. To realize this method, it is necessary to increase the output of the laser diode 1 or increase the recording sensitivity of the disk D as a recording medium. Therefore, there inevitably is a limit to the improvement of the recording speed.
More specifically, increasing the number of rotations of a disk is limited to about five times the current level due to the restriction that the objective lens for reading recorded information from the disk should accurately follow up the surface vibration of the disk, and improving the information density by shortening the wavelength to be read is limited to as high as two times the current density even with the use of blue laser light. That is, those two schemes, even when combined, will not improve (increase) the transfer rate more than ten times the current level.
As comparative methods, a scheme is described in Japanese Patent Kokai No. S 59-65948, and a three beam high vision disk system is proposed in the Japanese Patent Kokai No. H 2-185728 or in Institute of Television Annual Convention 1993 (20-1), ITEC '93: ITE Annual Convention, "Development of High Vision Video Disk System by Three Beam Reading System". Those methods simultaneously irradiate three laser beam spots SP1, SP2 and SP3 on three adjacent tracks on a disk to record or read information on or from the three tracks at the same time as shown in FIGS. 2A and 2B, thereby improving the transfer rate.
If those methods are used, however, a disk designed for the methods has triple spiral tracks. In other words, rows of spiral recording pits which are equal in number to the beam spots are formed. The disk having information recorded in that manner therefore requires an optical head, which generates the same number of light beam spots also for reproduction, and its information cannot be reproduced by an existing player which uses only one beam.